Processes and compositions for conditioning phosphates



United States Patent ABSTRACT OF THE DISCLO SU RE A process is disclosedfor manufacturing calcium disodium pyrophosphate tetrahydrate of whichmore than 50 weight percent is amorphous which process comprisesreacting sodium acid pyrophosphate and calcium hydroxide in an aqueousmedium at a pHfrom about 6 to about 10.5. The foregoing composition isuseful as an anticaking agent for monocalcium orthophosphatemonohydrate.

This is a division of my co-pending application Ser. No. 125,947, filedJuly 24, 1961, now US. Patent 3,244,478.

This invention relates to processes for reducing the normal tendency ofphosphatic materials to cake, and to novel phosphatic compositions thatresist caking.

The tendency of particulated (powdered, granular, or flaked) monocalciumorthophosphate monohydrate (MCP) to cake, or to form large, hard lumps,when it is exposed to a relatively high humidity for an extended periodof time, is well known. Several attempts have been made to eliminatethis undesirable tendency by utilizing procedures which were known tohave solved similar problems with respect to other hygroscopic salts.However, to date, none of these has been considered completelysuccessful.

It is an object of this invention to make available a stabilizedmonocalcium orthophosphate monohydrate which upon exposure to relativelyhigh humidity conditions, resists the normal tendency of monocalciumorthophosphate monohydrate to cake.

It is another object of this invention to provide a method whereby anacceptably flow-conditioned monocalcium orthophosphate monohydrateproduct can be produced.

It is still another object of this invention to make available a newform of calcium disodium pyrophosphate tetrahydrate which isparticularly useful as an anti-caking agent for monocalciumorthophosphate monohydrate.

It is still another object of thisinvention to provide a new method forthe production of calcium disodium pyrophosphate and its tetrahydrate.

The first two of the above objects, as well as others, can be achievedby blending into particulated monocalcium orthophosphate monohydrate(MCP) a small amount of a finely divided mixed cation pyrophosphate suchas, for example, calcium disodium pyrophosphate. The third and fourthobjects listed above can be accomplished by reacting (in an aqueousmedium) sodium. acid pyrophosphate with calcium hydroxide.

The term mixed cation pyrophosphate is intended herein to encompassinorganic pyrophosphates having the formula:

wherein M is an alkaline earth metal cation such as calcium, magnesium,strontium, etc. (and preferably either calcium or magnesium), and A isan alkali metal 3,409,393 Patented Nov. 5, 1968 "ice cation, andpreferably is sodium. Both the crystalline and the amorphous forms ofthe mixed cation pyrophosphates are effective in inhibiting the normalcaking tendencies of MCP. It is preferred, however, that a new andunexpected form of calcium disodium pyrophosphate (i.e. the amorphoustetrahydrate) be utilized.

While, in order to improve the caking properties of particulated MCP,very small amounts of finely divided mixed cation pyrophosphates areeffective, usually at least about 0.5 weight percent (base on the weightof the MCP) of the mixed cation pyrophosphates will ordinarily beinterspersed through the MCP in the practice of this aspect of theinvention. For optimum results in this application, usually at leastabout 0.75 weight percent of one of the finely divided mixed cationpyrophosphates should be mixed with the MCP. There is no critical upperlimitation as to the amount of mixed cation pyrophosphates that can beinterdispersed through the particulated MCP in the practice of theinvention. As a practical matter, however, usually not more than about10 weight percent is utilized. When one of the preferred mixed cationpyrophosphates are utilized, usually not more than about 7 weightpercent (base on the weight of MCP) of it is present in the inhibitedMCP compositions.

Apparently, some of the substantial benefits which are achieved byintermixing the mixed cation pyrophosphates into the MCP according tothe practice of this invention result from the adherence of theadditives onto the surfaces of the particles of MCP. Nevertheless, forexcellent anti-caking results, finely divided mixed cationpyrophosphates need only to be interspersed reasonably well through theMCP. Sufliciently good dispersion can be accomplished, for example, inpractically any conventional mixer or blender. The particulated MCP andthe mixed cation pyrophosphate need only be intermixed in theappropriate proportions in a conventional ribbon-type mixer, forexample, for about 5 minutes in order to achieve an excellent degree ofdispersion of the anticaking additive through the-MCP.

The term finely divided, as applied to the monoalkaline earth metaldisodium pyrophosphates in this specification and the appended claimsmeans those mixed cation pyrophosphate products having sufiicientlysmall particles to pass largely -(i.e. at least about weight percent)through a US. Standard 325-mesh screen. The particles are smaller thanabout 44,41. in diameter. With respect to the preferred amorphous formof the calcium disodium pyrophosphate tetrahydrate (which will bedescribed in greater detail below), the average particle size isordinarily inherently much smaller than 4411.. For example, theamorphous tetrahydrate has been found to be extremely effective when itsaverage particle size (diameter) is below about 7-10 microns, whichparticle size usually results directly from the processes of thisinvention described below.

In order to produce calcium disodium pyrophosphate which is largely inthe amorphous form (that is, more than about 50 Weight percent, andusually more than 70 weight percent is amorphous, as determined byconventional X- ray diffraction techniques), the preferred process is anunexpected one; the reaction of sodium acid pyrophosphate (Na H P O inan aqueous medium with calcium hydroxide. The reaction which is utilizedin this aspect of the invention is:

calcium hydroxide in the above reaction is such that upon itscompletion, the reaction will result in a pH of the aqueous mediumbetween preferably between about 7 and about 10. Final pHs within thesedesirable ranges can result when the molar ratio of sodium acidpyrophosphate to calcium hydroxide is between about 0.7 and about 1.3,and preferably between about 0.85 and about 1.1. When a manufacturer ofthe novel calcium disodium pyrophosphate of this invention desires tomaintain the amount of water-soluble material in the calcium disodiumpyrophosphate product at a minimum, he must charge (into the particularaqueous media employed) his raw materials at molar ratios (of sodiumacid pyrophosphate to calcium base) between about 0.85 and about 1.0.

The temperature of the reaction media during the abovedescribed reactionapparently has no effect whatever upon the excellent performance of thefinal calcium disodium pyrophosphate product as an anti-caking agent forMCP. In consideration of the effect of such factors as Viscosity andslurry concentration, and the fact that a small amount of heat isevolved during the reaction, the aqueous reaction media are preferablyat a temperature between about 20 C. and about 85 C. before the reactionis begun. While relatively higher temperatures result in higher rates ofreaction, the actual rate of reaction, too, has apparently no effectupon the performance of the resulting largely amorphous calcium disodiumpyrophosphate products as anti-caking agents for MCP.

Nor does the order of addition of the sodium acid pyrophosphate andcalcium hydroxide (or calcium oxide) to the reaction vessel have anynoticeable effect upon the physical characteristics of the product whichresults, or upon its overall performance in this invention. The calciumoxide or hydroxide can, for example, be first slurried into water. Thenthe resulting slurry can be intermixed wvith the proper amount of sodiumacid pyrophosphate in a suitable conventional reaction vessel. Thesodium acid pyrophosphate can be predissolved in a small amount of waterif desired. Or, if desired, particulated sodium acid pyrophosphate canbe premixed with the calcium oxide or hydroxide before the resultingblend is slurried into 'water in the reaction vessel. As anotherexample, sodium acid pyrophosphate can be predissolved in water. Thenthe calcium hydroxide or oxide can be added to the sodium acidpyrophosphate solution in the reaction vessel.

Since it dissolves readily in the aqueuos media employed in thisprocess, the particle size of the sodium acid pyrophosphate which isutilized in this invention is not at all critical. The particles of thewater-insoluble calcium bases which are utilized, however, must befairly small in order for the reaction (with sodium acid pyrophosphateto be completed within a fairly short period of time. For example, whencalcium oxide or calcium hydroxide are reacted with sodium acidpyrophosphate, the reaction is complete after only about 15 minutes ifsubstantially all of the particles of the calcium base are small enoughto pass through a U.S. Standard ZOO-mesh screen. By comparison, if theparticles of calcium hydroxide, for example, are so large that thegreater proportion of them cannot pass through a U.S. Standard 100-meshscreen, more than 45 minutes may be required for this material to reactcompletely with an equivalent amount of sodium acid pyrophosphate. It ispreferred, therefore, that in the practice of this invention theparticles of the particular water-insoluble calcium base which isutilized be so small that at least 80 weight percent of them can passthrough a U.S. Standard ZOO-mesh screen.

Because calcium hydroxide is insoluble in water, the reaction of sodiumacid pyrophosphate with it will be taking place in what appears to be aslurry, or a fluid paste. As the reaction proceeds, the viscosity ofthis reaction slurry increases to a point at which the slurry becomesalmost immobile, and cannot be agitated except with very high-poweredmixing equipment unless the reacting solids (proportion of the slurrywhich is not volatile at about 6 and about 10.5, and" r r e 3': 110 C.)are'-maintained"below about' 35 Weight percent. For this reason it ispreferred that, where ones mixing equipment is not capable of handlingvery viscous fluids, for example, the reaction slurry solids bemaintained below about 25 weight percent. Concurrently, the water levelin the slurry should beat least about weight percent and preferably atleast about weight percent of the slurry. After the very viscous'stagein the above-described reaction (which generally occurs aboutmidway inthe reaction period) the; viscosity of the slurry decreasesconsiderably, and no further diffi'culty' with viscosityj isencounteredthrough the remainder of the reaction period.

After the reaction of the sodium.acid pyrophosphate with the calciumhydroxide has progressed sutficiently (so that the pH of the slurry isconstant and within the desired range), the water is removed by anyappropriate procedure desired. The only precaution that one need observeduring the removal of the water from the slurry of water and the calciumdisodium pyrophosphate tetrahydrate product (which resulted from thereaction described here tofore) is that if one desires to preserve theproduct in the form of the tetrahydrate, he should not permit thetemperature of the product from the above-described reaction to riseabove about 130 C. for an extended period of time.'Slurries containingthe largely amorphous calcium disodium pyrophosphate tetrahydrate ofthis invention can be drum-dried, for example, or even spray-dried,utilizing conventional techniques, without substantial loss of theproducts water of crystallization, provided the temperature of thispreferred mixed cation pyrophosphate is maintained below about 130 C.Also, the slurry can be filtered, and the filter cake washed, if desired(to remove water-soluble salts that might be contained therein) beforeit is dried. The presence of reasonable amounts of impurities, such asunreacted sodium acid pyrophosphate and calcium hydroxide, in thecalcium disodium pyrophosphate tetrahydrate product, however, does notnoticeably decrease its effectiveness as an anti-caking agent. Forexample, the presence of 15 weight percent of calcium hydroxide and/orsodium acid pyrophosphate has apparently no detrimental effect whateverupon the ability of largely amorphous calcium disodium pyrophosphate toreduce the tendency of MCP to cake. I;

When the calcium disodium pyrophosphate is dried, its individualparticles sometimes loosely stick together forming flakes or granuleswhich are larger than desired for its efficient performance as ananti-caking agent for MCP. When such agglomeration occurs, a single passthrough a conventional powder grinder such as, for example, aconventional hammer mill is usually suflicient to produce a producthaving the desired small particles. If the grinding of the dried productis objectionable, it has been found that the presence of at least about15 Weight percent (based on the weight of the calcium disodiumpyrophosphate) of a finely divided (more than weight percent can bepassed through a U.S. standard IQO-mesh screen) inorganic siliceousmaterial such as sodium silico aluminate, calcium aluminum silicate,etc., in the slurry during the drying step causes the dried product toremain in a very finely divided state, thus often eliminating thenecessity for. the above-described grinding step. The presence of evenas much as 60 weight percent of the finely divided inorganic siliceousmaterial in the calcium disodium pyrophosphate apparently has nodetrimental effect upon its performance as an anti-caking agent for MCP.

While it can be observed that the above discussion has been directedlargely toward the preferred new form of calcium disodium pyrophosphatetetrahydrate (the amorphous form) it should be noted that the inventionis not limited to the use of the amorphous form of the tetra hydrate asan anti-caking agent. Most of the substantial benefits of this inventioncan also be observed when finely divided anhydrous amorphous calciumdisodium pyrophosphate and/or the crystalline form of the tetrahydrate,

as well as magnesium disodium pyrophosphate, calcium dipotassiumpyrophosphate, magnesium dipotassium pyrophosphate, etc. are intermixedwith MCP.

Nor is the utility of the novel largely amorphous calcium disodiumpyrophosphate of this invention confined to that of an anti-cakingagent. Other uses for which this compound is peculiarly suited includeits application as a mild abrasive in dentifrices, as an inertinsecticide, and as a concentrated source of calcium and phosphate inthe fields of human and animal nutrition.

In the following examples which are illustrative of some of thepreferred embodiments of this invention, all parts are by weight unlessotherwise specified. Example I illustrates one procedure for preparingcalcium disodium pyrophosphate tetrahydrate which is largely in theamorphous form; that is, which contains most of the calcium disodiumpyrophosphates in the amorphous state.

EXAMPLE I Into a conventional stainless steel mixing tank fitted with animpeller-type stirrer are poured 31,180 parts of water, 5,820 parts ofsodium acid pyrophosphate, and 2,000 parts of commercial hydrated lime[Ca(OH) The molar ratio of sodium acid pyrophosphate to calciumhydroxide in this mixture is about 0.97. The resulting slurry is thenheated to 50 C. Within about minutes the pH of the slurry has becomesteady at about 9. Small final adjustments of the slurry pH to thedesired level are made by the addition of small quantities of lime orsodium acid pyrophosphate. The reaction is complete when the pH of theslurry has remained steady for about onehalf hour after the lastaddition of raw materials.

After the reaction of sodium acid pyrophosphate and calcium hydroxide iscompleted, the water in the slurry is driven off by drying the slurry ona conventional 36-inch diameter splash-feed stainless steel dmm dryerunder an 80 pound head of steam, at a drum speed of 6 rpm. The resultingdried product contains about weight percent of water as water ofhydration, and is largely amorphous (i.e., weight percent is amorphous),according to X-ray spectrographic analysis. It contains many looselyagglomerated lumps, which are easily broken when the dried product ispassed once through a conventional ham- I mer mill.

The beneficial use (as an anti-caking agent for MCP) of calcium disodiumpyrophosphate tetrahydrate produced in this example is illustrated inExample II.

EXAMPLE II One thousand parts of powdered commercial monocalciumorthophosphate monohydrate are blended in a conventional ribbon-typemixer for about 15 minutes with 15 parts of the finely divided amorphouscalcium disodium pyrophosphate tetrahydrate which has been produced asin Example I, above. Thirty grams of the resulting blend are placedloosely in a 3 cm. by 3 /2 cm. rigid-walled cylinder and are thencovered (top and bottom) with moisture-permeable .cloth. The sample isthen held for 48 hours at 90 F. in an 80 percent relative humidityatmosphere and under 6.7 psi. (gauge) pressure, in order to test itsresistance to caking. Table I lists the results of this test. Note thatthe results are given in terms of caking numbers, which represent thepounds of force, applied at a uniform rate of increase (one additionalpound of force applied each second) directly onto the entire top surfaceof the cake of MCP (after it is removed from the above-describedcylinder) by means of a 3.5 cm. diameter fiat disc. The caking numbersare average values from analyses made in triplicate. Table I alsocontains caking number data for other forms of calcium disodiumpyrophosphate, for magnesium disodium pyrophosphate, and for materialswhich are known to function as anti-caking agents or flow conditionersfor materials other than monocalcium orthophosphate monohydrate (MCP).

TABLE L-OAKING NUMBERS FOR VARIOUS ANTI- CAKING AGENTS 1 Averageparticle size= 10 Made by reacting sodium acid pyrophgspriiigtfilyvithcalcium chloride, filtering, washing with water, drying, anl A gent bfdried at 250 C. to remove water of crystallization. Average particlesize= 10 3 Same as agent b., except made with M8012. Average particlesize Ai erage particle size= 10 5 Average particle size 1n.

Dusty.

Note that the above test is a very severe one, and that as a rule, thoseanti-caking agents that give caking numbers less than about 20 are atleast commercially acceptable, and would be expected to perform at leastreasonably satisfactory as anti-caking agents for MCP under ordinaryconditions of shipping, storage, and handling. The above dataillustrates the unexpected superiority of the compositions and processesof this invention over products which are presently commerciallyrecommended and used for this purpose.

What is claimed is:

1. A process for manufacturing calcium disodium pyrophosphatetetrahydrate, of which more than 50 weight percent is amorphous, whichprocess comprises reacting together in an aqueous medium sodium acidpyrophosphate and calcium hydroxide at pH of from about 6 to about 10.5.

2. A process as in claim 1, wherein the molarratio of said sodium acidpyrophosphate to said calcium hydroxide is between about 0.7 and about1.3.

3. A process for manufacturing calcium disodium pyrophosphatetetrahydrate, of which more than 50 weight percent is amorphous,suitable for use as an anti-caking agent in monocalcium orthophosphatemonohydrate, which process comprises the steps of forming an aqueousslurry by intermixing water, sodium acid pyrophosphate, and calciumhydroxide in such amountsthat the molar ratio of said sodium acidpyrophosphate to said calcium hydroxide in said slurry is between about0.85 and about 1.1, and the amount ofwater in said slurry is equal to atleast about 65 weight percent of said slurry, allowing said sodium acidpyrophosphate to react with said calcium hydroxide, at a pH of fromabout 7 and about 10, and thereafter removing said water from theresulting reaction product at a temperature below about 130 C.

4. Calcium disodium pyrophosphate tetrahydrate, of which more than 50weight percent is amorphous.

5. A process for manufacturing a mixed cation pyrophosphate having theformula MNa P o suitable for use as an anti-caking agent wherein M is analkaline earth metal cation selected from the group consisting ofcalcium and magnesium, which process comprises reacting in an aqueousmedium sodium acid pyrophosphate with a base selected from the groupconsisting of calcium hydroxide and magnesium hydroxide at a molar ratioof said sodium acid pyrophosphate to said base of from about 0.7 toabout 1.3, intermixing with the resulting slurry from about 15 to about60 weight percent (based on the weight of said mixed cationpyrophosphate in said slurry) of a finely divided inorganic siliceousmaterial; at least about weight percent of the particles of said finelydivided inorganic siliceous material being small enough to pass througha US. Standard 100 mesh screen, drying the resulting mixture, andrecovering a composition containing being finely divided after saiddrying step without the necessity for subsequent grinding. p

6.=A composition which is suitable for use as an anticaking agent formono'calcium orthophosphatemonohydrate, which composition contains amixed cation pyrophosphate having the formula MA P oflwhe'rein M is analkaline earth metal cation and A is an alkali metal cation and havingan average particle size below about 44 microns and less than about 60Weight percent (based on the weight of said mixed cation py rophosphate)of a finely divided inorganic siliceous material; at least about 80weight percent of the particles of said finely divided inorganicsiliceous material beingsmall enough to pass through a U.S. Standard 100mesh screen whereby the tendency for the particles of said mixed cationpyrophos- 20 phate to adhere together is reduced.

' 7. A composition as in claim 6, wherein said mixed 8 8. A compositionas in claim 7, wherein at least 50 percent of said calcium disodiumpyrophosphate tetrahydrate is amorphous. 9. A composition as in claim 8,wherein said inorganic siliceous material is sodium silico 'aluinin'ate.

A References Cited v v UNITED STATESPATENTS.

2,287,699 6/1942 Moss et a1. 23-109 OS CAR R. VERTIZ-JPrimary Examiner.

cation pyrophosphate is calcium disodium pyrophosphate. L. A. MARSH,Assistant Examiner.

